The subject matter disclosed herein relates generally to compressors. More particularly, the subject matter disclosed herein relates to electromagnetically driven reciprocating compressors adapted for use in displacing fluids, such as oil or natural gas.
Reciprocating compressors are widely used in the oil and gas industry to pressurize and displace gas. For example, in gas pipeline transmission systems and distribution networks, reciprocating compressors move natural gas from production sites to end-users by ingesting relatively low pressure gas, and expelling the gas at a higher pressure. Reciprocating compressors also perform this same function used in industrial plants, such as petroleum refineries and chemical plants, where compressors move intermediate and end product gases.
Reciprocating compressors typically include a piston driven by a rotatory motor, such as an internal combustion engine or motor. In such systems, a crankshaft and connecting rod convert motor shaft rotation to piston translation in a compression chamber. Piston translation within a cylinder bore in turn compresses gas in a compression chamber located at an end of the cylinder bore. Such machines may be single action, where gas compression takes place only when the piston moves in a single direction, or double action, where gas compression takes place when the piston moves in two directions.
Rotary reciprocating compressors have several disadvantages.
First, during the majority of each rotation of the motor shaft, the connecting rod applies force to the piston at an angle with respect to the piston translation axis.
Since the crankshaft is mechanically linked piston, piston travel during each stroke is fixed. Therefore, the volume swept by the piston during a stroke is also fixed. This means that the, in order to change the volume of gas pumped over time, operating speed must be changed. Operating speed change limits the flexibility of the machine insofar as pumping capacity as, in order to change the volume of gas pumped over time, the machine be sped up or slowed down, as would is necessary when gas demand in the distribution network increases or decreases. Changing operating speed is undesirable because it reduces efficiency and changes the vibration frequency imposed on the equipment.
One solution to these problems is an electromagnetically actuated reciprocating compressor. Such systems use linear motors attached to piston rods to drive opposed pistons in a single compression chamber. When the pistons move in phase with a 0 degree offset, thereby maintaining a fixed distance between the opposed pistons, the compression chamber volume remains constant, and reciprocation effects minimal gas displacement (or gas compression). When the pistons move out of phase with a 180 degree offset, thereby minimizing compression chamber volume when the pistons reach top dead center, and minimizing compression chamber volume when the pistons reach bottom dead center, reciprocation alternately minimizes and maximizes volume to effect maximum gas displacement (or gas compression). Varying the phase angle between these two extremes therefore provides a means for varying displacement (and compression) from a minimum when the pistons move “in phase”, and a maximum when the pistons move “out of phase”.
Unfortunately, currently available linear motor technology is unsuitable for use in such phased compressors because the associated high inertial rod loads. Existing linear motors can generate a limited amount of force, and the inertia associated with piston rod/compression piston assemblies in machines suitable for use in natural gas systems exceeds that available from existing linear motors. In addition, oppositely arranged pistons in a standard reciprocating compressor would make the machine prohibitively large. And varying the phase between oppositely arranged pistons in a standard reciprocating compressor is not an easy or fast operation.
Accordingly, there is a need for an electromagnetic actuator for a piston rod where phasing control can be easily obtained by controlling current command on the electromagnetic motor. There is a further need for an electromagnetic actuator that allows for a compact machine. Finally, there is a need for an electromagnetic actuator that can overcome the high inertial forces associated with accelerating and decelerating a piston rod/compression piston assembly.